The present invention relates to a process for the production of fatty acid mixtures containing a high proportion of C.sub.6 -C.sub.10 -fatty acids.
Esters of naturally occurring fatty acids and short-chain aliphatic alcohols, particularly fatty acid methyl esters, are of considerable commercial significance, above all as starting materials for the production of fatty alcohols and derivatives thereof. However, they are also used for the production of other oleochemical products, for example fatty acid alkanolamides, fatty acid ester sulfonates and soaps. In view of the performance properties required of the end products, alkyl esters of fatty acids containing from 12 to 18 carbon atoms, particularly in the form of mixtures, are of particular interest for the abovementioned applications. Fatty acid ester mixtures such as these are primarily obtained by the catalytic transesterification of fats and oils of natural origin, which are known to consist of fatty acid triglycerides, with the corresponding short-chain alcohols, above all with methanol.
When subjected to direct catalytic transesterification, most naturally occurring fats and oils give ester mixtures of which the fatty acid component consists of C.sub.12 -C.sub.18 -fatty acids. If coconut oil or palm kernel oil is used as starting material for transesterification, the ester mixtures obtained contain a high percentage of esters of short-chain fatty acids, the presence of which is generally undesirable. For this reason, the alkyl ester mixtures obtained from coconut oil and palm kernel oil by transesterification are generally separated by distillation into a low-boiling fraction, which contains the alkyl esters of the C.sub.6 -C.sub.10 -fatty acids (first runnings fatty acid alkyl esters), and a higher-boiling fraction which contains the alkyl esters of the C.sub.12 -C.sub.18 -fatty acids.
The main fraction of the coconut oil and palm kernel oil fatty acid alkyl esters serves as starting material for the production of fatty alcohols and fatty acid alkanolamides. Part of the first runnings fatty acid alkyl esters separated off is processed by highpressure hydrogenation into C.sub.6 -C.sub.10 -fatty alcohols. The remainder is split by hydrolysis into the free C.sub.6 --C.sub.10 -fatty acids which are then marketed as first runnings fatty acids, also known as feed fatty acids.
The first runnings fatty acid alkyl esters may be converted into the free fatty acids in known manner by alkaline hydrolysis and subsequent decomposition of the soaps with mineral acids or by acid hydrolysis in the presence of acidic catalysts.
Alkaline hydrolysis involves considerable outlay on chemicals because stoichiometric quantities of alkali metal hydroxide and mineral acids have to be used. In addition, the aqueous alkali metal salt solutions formed give rise to considerable effluent pollution.
The acid-catalyzed hydrolysis of the C.sub.6 --C.sub.10 -fatty acid alkyl esters is an equilibrium reaction which can only be continued to a satisfactory conversion level when water is used in a large excess. In this connection, particular problems are caused by the fact that the esters to be reacted dissolve very readily in the free fatty acids formed so that the velocity of the hydrolysis reaction is seriously reduced. Under practical conditions, satisfactory C.sub.6 -C.sub.10 -fatty acids can only be obtained when the fractions of alkyl esters still present at the end of hydrolysis are removed with steam. Also, aqueous alcohol solutions which have to be worked up by fractional distillation are again formed just as in the alkaline hydrolysis of the C.sub.6 -C.sub.10 -fatty acid esters.